The production of oxygen from air currently uses vacuum pressure swing adsorption (VPSA) or pressure swing adsorption (PSA) systems technology. These systems often have a capacity of <200 tons per day O2. Presently, there is renewed interest in extending the capacity of VPSA or PSA systems from such small scale (<200 tons per day O2) to large scale (about 350 tons per day O2 or higher) oxygen production from air.
In the application of VPSA or PSA processes, the energy input required to achieve the separation of O2 from the feed mixture (e.g., air) is provided as mechanical work through feed compressor(s) and vacuum pump(s). The cost of this work is a significant component of the total operating cost of the VPSA or PSA process. In addition, VPSA or PSA technology is currently economically competitive with cryogenic distillation only for small scale applications. In order for PSA or VPSA processes to become cost competitive with cryogenic distillation for large scale applications, improved cycles are required to operate the PSA or VPSA processes.
Two bed vacuum pressure swing adsorption (VPSA) processes for the production of oxygen from air are disclosed U.S. Pat. No. 5,518,526 to Baksh et al. and U.S. Pat. No. 6,010,555 to Smolarek et al. U.S. Pat. Nos. 5,518,526 and 6,010,555 employ VPSA processes with simultaneous equalization and evacuation steps followed by simultaneous feed and product pressurization steps. FIG. 1 shows the VPSA cycle for the production of oxygen from air disclosed in Smolarek et al., U.S. Pat. No. 6,010,555. U.S. Pat. Nos. 5,518,526 and 6,010,555 are both for small scale (<200 tons per day, (TPD)) oxygen production. Adsorption and desorption pressures in U.S. Pat. Nos. 5,518,526 and 6,010,555 are characterized by a low pressure ratio and relatively high desorption pressure values. Significant reduction in equipment and operating costs can be realized using the small scale (<200 TPD O2) VPSA processes of U.S. Pat. Nos. 5,518,526 and 6,010,555.
In applications where large scale oxygen production (e.g., 350 tons per day O2) is desirable, four bed VPSA processes are used. One such VPSA process is disclosed by Smolarek et al., U.S. Pat. No. 5,656,068. The four bed VPSA process disclosed in U.S. Pat. No. 5,656,068 is operated as two pairs of 2-bed systems, referred to as a 2×2 cycle/system. Each pair of beds is operated 180° out of phase and the two pairs of beds are operated out of phase by one-half of a half-cycle. Two compressors (one Roots or positive displacement and one Centrifugal) and two vacuum pumps (one Roots or positive displacement and one Centrifugal) are disclosed in the VPSA process of U.S. Pat. No. 5,656,068. One of the two compressors is periodically in the idle or vent mode. A four bed VPSA system operating as two pairs of adsorption beds to produce about 100 tons per day (TPD) oxygen is disclosed in Doong, U.S. Pat. No. 5,997,612 to Doong. The VPSA process includes two pairs of beds, an intermediate storage tank (to collect co-current depressurization gas), one gas blower and a pair of vacuum pumps. The system disclosed in Doong (U.S. Pat. No. 5,997,612) includes three pumps as compared to the four pumps disclosed in U.S. Pat. No. 5,656,068. In addition, the system shown in Doong (U.S. Pat. No. 5,997,612) is for small scale oxygen production (<200 tons per day) rather than large scale O2 production using the dual feed VPSA process of Smolarek et al. (U.S. Pat. No. 5,656,068).
It would thus be desirable to provide cost efficient, highly productive four bed VPSA processes and systems with the capacity for large scale oxygen production (e.g., >350 tons/day O2).